The Collaborative International Dictionary of English v.0.48:

Lumber \Lum"ber\, n. [Prob. fr. Lombard, the Lombards being the
   money lenders and pawnbrokers of the Middle Ages. A lumber
   room was, according to Trench, originally a Lombard room, or
   room where the Lombard pawnbroker stored his pledges. See
   Lombard.]
   1. A pawnbroker's shop, or room for storing articles put in
      pawn; hence, a pledge, or pawn. [Obs.]
      [1913 Webster]

            They put all the little plate they had in the
            lumber, which is pawning it, till the ships came.
                                                  --Lady Murray.
      [1913 Webster]

   2. Old or refuse household stuff; things cumbrous, or bulky
      and useless, or of small value.
      [1913 Webster]

   3. Timber sawed or split into the form of beams, joists,
      boards, planks, staves, hoops, etc.; esp., that which is
      smaller than heavy timber. [U.S.]
      [1913 Webster]

   Lumber kiln, a room in which timber or lumber is dried by
      artificial heat. [U.S.]

   Lumber room, a room in which unused furniture or other
      lumber is kept. [U.S.]

   Lumber wagon, a heavy rough wagon, without springs, used
      for general farmwork, etc.

   dimensional lumber, lumber, usually of pine, which is sold
      as beams or planks having a specified nominal
      cross-section, usually in inches, such a two-by-four,
      two-by-six, four-by-four, etc.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Lumber \Lum"ber\, v. t. [imp. & p. p. Lumbered; p. pr. & vb.
   n. Lumbering.]
   1. To heap together in disorder. " Stuff lumbered together."
      --Rymer.
      [1913 Webster]

   2. To fill or encumber with lumber; as, to lumber up a room.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lumber \Lum"ber\, v. i.
   1. To move heavily, as if burdened.
      [1913 Webster]

   2. [Cf. dial. Sw. lomra to resound.] To make a sound as if
      moving heavily or clumsily; to rumble. --Cowper.
      [1913 Webster]

   3. To cut logs in the forest, or prepare timber for market.
      [U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

lumber
    n 1: the wood of trees cut and prepared for use as building
         material [syn: lumber, timber]
    2: an implement used in baseball by the batter [syn: baseball
       bat, lumber]
    v 1: move heavily or clumsily; "The heavy man lumbered across
         the room" [syn: lumber, pound]
    2: cut lumber, as in woods and forests [syn: log, lumber]

Moby Thesaurus II by Grady Ward, 1.0:

225 Moby Thesaurus words for "lumber":
   amble, barge, be all thumbs, beam, beams, billet, blunder,
   blunder away, blunder into, blunder on, blunder upon, board,
   boarding, boards, boggle, botch, bowl along, bumble, bundle,
   bungle, burden, burthen, butcher, cargo, charge, chug on,
   clamjamfry, clapboard, clog, clump, clutter, commit a gaffe, cord,
   cordwood, cramp, cripple, cross, cumber, cumbrance, deadweight,
   deal, debris, difficulty, disadvantage, drag, driftwood, dust,
   embarrass, embarrassment, encumber, encumbrance, enmesh, ensnarl,
   entangle, entoil, entrammel, entrap, entwine, faux pas, fetter,
   firewood, flounce, flounder, foot, footslog, freight, fumble, halt,
   hamper, hamstring, handicap, hardwood, hippety-hop, hitch, hobble,
   hop, impede, impediment, impedimenta, impose upon, imposition,
   inconvenience, involve, jog, jog on, jolt, jumble, jump, junk,
   lade, lame, land, lath, lathing, lathwork, lime, limp, litter,
   load, log, lunge, lurch, mar, mince, miscue, muddle, muff, murder,
   net, odds and ends, onus, overload, pace, pack, paddle, panelboard,
   paneling, panelwork, peg, penalty, piaffe, piaffer, plank,
   planking, planks, play havoc with, plod, plod along, plug,
   plug along, plyboard, plywood, pole, post, prance, press down,
   puncheon, rack, raff, rejects, riffraff, roll, rub on, rubbish,
   rubble, saddle, saddle with, sashay, saunter, schlep, scrap, scuff,
   scuffle, scuttle, shackle, shake, shamble, sheathing,
   sheathing board, sheeting, shingle, shoddy, shuffle, sideboard,
   siding, sidle, single-foot, skip, slab, slat, slink, slip, slither,
   slog, slouch, snarl, softwood, splat, spoil, stagger, stalk, stamp,
   stave, stick, stick of wood, stomp, stovewood, straddle, straggle,
   stride, stroll, strut, stumble, stump, swagger, swing, tangle, tax,
   three-by-four, timber, timbering, timberwork, tittup, toddle, toil,
   totter, traipse, trammel, tramp, trash, trip, trouble, truck,
   trudge, two-by-four, waddle, wamble, weatherboard, weigh,
   weigh down, weight, white elephant, wiggle, wobble, wood